Pointers

1
Pointers
2
Pointers and Address

• Pointer variables
• Normal variables contain a specific value (direct
  reference). ex count
• Pointers contain address of a variable that has a
  specific value (indirect reference). Ex countPtr

3
Pointers and Address

• Pointer declarations
• used with pointer variables
• int myPtr (pointer to an int )
• Multiple pointers require using a before each
  variable declaration
• int myPtr1, myPtr2
• Initialize pointers to 0, NULL, or an address
• 0 or NULL points to nothing (NULL preferred)

4
Pointers and Address

• Two important operators( and )
• (address operator)
• returns address of the variable
• int y 5
• int yPtr
• yPtr y // yPtr gets address of y
• yPtr points to y

yptr
500000
600000
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Pointers and Address

• (indirection/dereferencing operator)
• Returns a synonym/alias of what its operand
  points to
• yptr returns y (because yptr y)
• can be used for assignment
• yptr 7 (the same as y 7)
• and are inverses
• y ? y, yptr ? yptr

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Pointer and Function Arguments

• Call by value
• void swap(int x, int y)
• int t
• tx
• x y
• y t
• a 10, b 20
• swap(a, b)

main
swap
t
a
10
x
10
b
copy
20
y
20
swap cant work correctly
7

• Call by reference
• void swap(int x, int y)
• int t
• tx
• x y
• y t
• a 10, b 20
• swap(a, b)

main
swap
copy the address
t
a
10
x
a
b
20
y
b
x ? the alias of a y ? the alias of b
8
Pointer Arithmetic

• Arithmetic operations can be performed on
  pointers
• Increment/decrement pointer ( or --)
• note /-- has higher precedence than
• (i) ! i
• i ? i, i i 1 (i) ? increase the i
• Add an integer to a pointer( or , - or -)
• Pointer comparison ( lt, , gt )
• See which pointer points to the higher numbered
  array element
• Also, see if a pointer points to 0(NULL)
• if (ptr NULL)

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Pointer Arithmetic

• Pointers of the same type can be assigned to each
  other
• If not the same type, a cast operator must be
  used
• int a (int )b, where b is a type of void
• Exception pointer to void (type void )
• Generic pointer, represents any type
• No casting needed to convert a pointer to void
  pointer
• void pointers cannot be dereferenced

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Pointers and Arrays

• Arrays and pointers closely related
• Array name like a constant pointer
• int v5 (v can be seen as int )
• int vPtr v (vPtr v0)
• Pointers can do array subscripting operations
• vi vPtri (vi) (vPtri)

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Pointers and Arrays

• The index can be negative number.
• int a5 4, 3, 2, 1, 0
• int pt a 1 // The same as pt a1
• printf("d\n", pt-1)
• The output is 4

12
Character pointers and Functions

• A string is a character array and it is
  terminated with the null character \0
• Two kinds of declaration of string
• char aString test
• Char pString test

aSting
Test\0
pSting
Test\0
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Case Study String copy

• Strcpy copy string t to string s
• void strcpy(char s, char t)
• int i0
• while((si ti) ! \0)
• i

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Case Study String copy

• Using pointer to perform this task
• void strcpy(char s, char t)
• while((s t) ! \0)
• s
• t

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Case Study String copy

• Because has higher precedence, we can rewrite
  as following
• void strcpy(char s, char t)
• while((s t) ! \0)

• \0 is equal to the integer 0
• void strcpy(char s, char t)
• while (s t)

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Array of pointers

• Arrays can contain pointers
• Example array of strings
• char suit 4 "Hearts", "Diamonds",
  "Clubs", "Spades"
• suiti is an pointer, suitij is a char
• suit11 i

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Array of pointers

• Simple example command-line arguments
• int main(int argc, charargv)
• argc number of arguments
• argv a string array contains the arguments
• arguments are separated by blanks.
• argv0 is the name by which the program invoked.
• dir /w ? argv0 dir, argv1 /w

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Array of pointers
main(int argc, char args) int i
for(i1 I lt argc i) printf(ss,
argvi, (i lt (argc -1))? )
printf(\n)
At command line Cgt echo Hello, world. Hello,
world. Cgt
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Pointers to Functions

• Pointer to function
• Contains address of function
• Similar to how array name is address of first
  element
• Function name is starting address of code that
  defines function
• Function pointers can be
• Passed to functions
• Stored in arrays
• Assigned to other function pointers

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Pointers to Functions

• Declaration
• Declare directly
• int (foo)(char, int ) means that pointer to a
  function which takes a char and an int as
  arguments and returns an int
• nore compare with int foo(char int )
• Embedded in function declaration
• int foo(char a, int b) ? foo is a type of
• int (foo)(char, int )

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Pointers to Functions

• Call via function pointer
• int (foo)(char, int ) ? foo is a pointer to
  function, foo is the function
• ?(foo)(char1, i) is used to call the function

int add(int a, int b) return(ab) main()
int (foo)(int, int) foo add printf("d\n",
(foo)(4,5))
The output will be 9
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Pointers to Functions

• An example qsort
• in stdlib.h
• protype
• void qsort(void base, size_t nmemb, size_t
  size, int(compare)(const void , const void ))
• base pointer of the array
• nmemb array size
• size size of each element in array
• compare a compare function

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Pointers to Functions

• Just define compare function for your purpose,
  you can sort anything.
• For example, compare function for string, float,
  and integer.